Method and Apparatus for Generating OFDM Signals

ABSTRACT

A method in a transmitter circuit of generating a signal comprising a first sequence of OFDM symbols, which are to be transmitted within a frequency sub band of a second sequence of OFDM symbols is disclosed. A first cyclic prefix (CP) of the second sequence of OFDM symbols has a first duration, and a second CP of the second sequence of OFDM symbols has a second duration. In order to generate both the first and the second cyclic prefix with an integer number of equidistant samples, a first sampling rate is required. The method comprises generating the signal comprising the first sequence of OFDM symbols at a second sampling rate, lower than the first sampling rate, and adjusting a sampling phase during CPs.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/519,979, filed 23 Jul. 2019, which is a continuation of U.S. patentapplication Ser. No. 15/963,507, filed 26 Apr. 2018, which is acontinuation of U.S. patent application Ser. No. 15/503,882, filed 14Feb. 2017, which is a national stage entry under 35 U.S.C. § 371 ofinternational patent application serial no. PCT/EP2017/051857, filed 27Jan. 2017, which claims priority to U.S. provisional application Ser.No. 62/295,358, filed 15 Feb. 2016. The entire contents of each of theforegoing applications is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to an OFDM transmitter and a relatedmethod.

BACKGROUND

Intercommunication between machines and communication between machinesand humans, Machine type communication (MTC), are growing in importance.The networks of devices exchanging information are constantly growing inimportance and builds up the Internet of Things (IoT). Cellulartechnologies adopted for these particular MTC applications are beingdeveloped and have an important role within IoT. The requirements onkeeping cost and power consumption low are tough at the same time ascoverage needs to be extended. All these aspects need to be addressedwhen designing for the future access technologies of IoT. 3GPP (3rdGeneration Partnership Project) is currently investigating how to meetthese design objectives and several proposals are up for discussion. Thecurrent studies have recently been moved from GERAN to RAN and have thecommonality that they all have much lower system bandwidth compared toLTE (Long-Term Evolution) of today. All these so called “Clean slate”solutions target system bandwidth of around 200 kHz to enable deploymentin former GSM spectrum and on existing GSM sites.

One proposal is called NB IoT (Narrowband IoT). It should be noted thatNB IoT was initially referred to as NB LTE in early standardization, butwas later renamed to NB IoT. Other solutions are referred to asNarrowband (NB) M2M, and NB OFDMA. Also a merged solution called NB CIoT(cellular IoT) with NB M2M uplink and NB OFDMA downlink has beenproposed.

One feature of NB IoT is in-band operation, i.e., NB IoT can be deployedby puncturing LTE subcarriers one physical resource block (PRB) wide anduse it for NB IoT transmission. To enable this in-band operation, it isimportant to synthesize the NB IoT numerologies with legacy LTE to avoidmutual interference between NB IoT and legacy LTE. In particular, NB IoTis supposed to keep LTE time-domain structure including orthogonalfrequency-division multiplexing (OFDM) symbol duration and cyclic prefix(CP) duration. A straight forward solution is to reuse the sample rateto system bandwidth relation of LTE, i.e. 1.92 MHz, 3.84 MHz, 7.68 MHz,15.36 MHz, 23.04 MHz, and 30.72 MHz sample rate related to 1.4 MHz, 3, 5MHz, 10 MHz, 15 MHz, 20 MHz LTE channel bandwidths.

SUMMARY

The inventors have developed an approach for obtaining signal generationat a relatively low sampling rate in NB IoT and similar systems.

According to a first aspect, there is provided a method in a transmittercircuit of generating a signal comprising a first sequence of an integernumber N_(sym) of OFDM symbols, which are to be transmitted within afrequency sub band of a second sequence of N_(sym) OFDM symbols. Thefirst sequence of OFDM symbols and the second sequence of OFDM symbolsare to be transmitted with the same timing. A first cyclic prefix of thesecond sequence of OFDM symbols has a first duration. A second cyclicprefix of the second sequence of OFDM symbols has a second duration. Thesecond duration is shorter than the first duration, such that in orderto generate both the first and the second cyclic prefix with an integernumber of equidistant samples, a first sampling rate is required. Themethod comprises generating the signal comprising the first sequence ofOFDM symbols at a second sampling rate, lower than the first samplingrate. The method further comprises adjusting a sampling phase duringcyclic prefixes.

According to some embodiments, a subcarrier spacing of the OFDM symbolsof the first and second sequences of OFDM symbols are 15 kHz, the firstduration is 160/30.72 μs and the second duration is 144/30.72 μs.

According to some embodiments, the first sampling rate is 1.92 MHz.

According to some embodiments, OFDM symbols of the first sequence ofOFDM symbols have 12 subcarriers.

According to some embodiments, the first cyclic prefix is an initialcyclic prefix of the second sequence of OFDM symbols, and all subsequentcyclic prefixes of the second sequence of OFDM symbols has the secondduration.

According to some embodiments, the second sequence of OFDM symbols aretransmitted in a Third-Generation Partnership Project (3GPP) Long-termEvolution (LTE) system and corresponds to a slot.

The first sequence of OFDM symbols may be transmitted in an NB IoTsystem.

According to some embodiments, the second sampling rate is 240 kHz.

According to some embodiments, the second sampling rate is 480 kHz.

According to some embodiments, the second sampling rate is 960 kHz.

According to some embodiments, adjusting the sampling phase comprisessetting an initial sample instant during an OFDM symbol to occur anon-integer multiple of periods at the second sampling rate after afinal sample instant of a preceding OFDM symbol.

According to a second aspect, there is provided a transmitter circuitfor generating a signal comprising a first sequence of an integer numberN_(sym) of OFDM symbols, which are to be transmitted within a frequencysub band of a second sequence of N_(sym) OFDM symbols. The firstsequence of OFDM symbols and the second sequence of OFDM symbols are tobe transmitted with the same timing. A first cyclic prefix of the secondsequence of OFDM symbols has a first duration. A second cyclic prefix ofthe second sequence of OFDM symbols has a second duration. The secondduration is shorter than the first duration, such that in order togenerate both the first and the second cyclic prefix with an integernumber of equidistant samples, a first sampling rate is required. Thetransmitter circuit comprises a digital-to-analog converter configuredto generate the signal comprising the first sequence of OFDM symbols ata second sampling rate, lower than the first sampling rate. Thetransmitter circuit also comprises a control unit configured to adjust asampling phase of the digital-to-analog converter during cyclicprefixes.

According to some embodiments, a subcarrier spacing of the OFDM symbolsof the first and second sequences of OFDM symbols are 15 kHz, the firstduration is 160/30.72 μs and the second duration is 144/30.72 μs.

According to some embodiments, the first sampling rate is 1.92 MHz.

According to some embodiments, OFDM symbols of the first sequence ofOFDM symbols have 12 subcarriers.

According to some embodiments, the first cyclic prefix is an initialcyclic prefix of the second sequence of OFDM symbols, and all subsequentcyclic prefixes of the second sequence of OFDM symbols has the secondduration.

According to some embodiments, the second sequence of OFDM symbols aretransmitted in a Third-Generation Partnership Project, 3GPP, long termevolution, LTE, system and corresponds to a slot.

The first sequence of OFDM symbols may be transmitted in an NB IoTsystem.

According to some embodiments, the second sampling rate is 240 kHz.

According to some embodiments, the second sampling rate is 480 kHz.

According to some embodiments, the second sampling rate is 960 kHz.

According to some embodiments, the control unit is configured to adjustthe sampling phase by setting an initial sample instant during an OFDMsymbol to occur a non-integer multiple of periods at the second samplingrate after a final sample instant of a preceding OFDM symbol.

According to a third aspect, there is provided an electronic devicecomprising the transmitter circuit according to the second aspect. Theelectronic device may e.g. be an MTC device or a network node for acellular communication system.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of examples of the disclosurewill appear from the following detailed description, reference beingmade to the accompanying drawings, in which:

FIG. 1 illustrates functional blocks of a transmitter.

FIG. 2 illustrates the output of a digital to analog converter.

FIG. 3 illustrates a method.

FIG. 4 is a block diagram of a transmitter.

FIG. 5 illustrates a communication environment.

DETAILED DESCRIPTION

One challenge for NB IoT is to be able to reuse the legacy LTE samplingrate relationship to system BW (bandwidth). There will not be an integernumber of samples per cyclic prefix symbol if scaling is applied justrelated to required number of samples per symbol. One solution is to isto perform a straight forward upsampling/downsampling to the LTE lowestsampling rate of LTE, 1.92 MHz, and do the CP removal and additionrespectively. The inventors have realized that this may be too expensivefor low cost MTC devices, since it requires more device processing andconsumes more power with decreased battery life and increased chipsetcost as consequence. It is crucial with low cost and low power for amany MTC applications and desirable to be able to reduce the samplingrate for the complete NB IoT system and not only partly.

This disclosure proposes a transmitter architecture for keeping completesystem sampling rate as low as possible and avoid any up-sampling tohigher clock frequency for CP addition. According to examples, adigital-to-analog converter (DAC) is configured with non-uniformsampling triggered when cyclic prefix is to be transmitted.

Although the cost of NB IoT eNodeB (i.e. on the network side) is of lessconcern than on the device side, the proposed transmit chains can alsobe used at the eNodeB to reduce cost wherever necessary.

One beneficial feature of NB IoT is the in-band operation with legacyLTE. This makes it preferred to more or less use the LTE numerology.Subcarrier spacing is 15 kHz and CP length 160/30.72 microseconds forfirst OFDM symbol of every slot and 144/30.72 microseconds for theother. The lowest required sampling rate applicable for an integernumber of samples per CP, for both CP lengths, is 1.92 MHz. At 1.92 MHz,the CP of the first OFDM symbol would be represented with 10 samples,and the CP of the other OFDM symbols would be represented with 9 sampleseach.

In this disclosure, an architecture, suitable for NB IoT, is proposed tofurther reduce the sample rate compared with the 1.92 MHz mentionedabove. The UE (User Equipment) is more cost sensitive than the eNodeBand the focus therefore is on the UE side. However, the disclosed ideasare applicable also on the eNodeB side.

NB IoT is used as an example system in this disclosure, but thedisclosure may be applied in other similar systems as well.Considerations of a transmitter design are presented below in terms ofan example.

The useful bandwidth of NB IoT is 180 kHz even when it is deployedwithin a wide LTE carrier. Therefore, it is enough to have 180 kHzsample rate to satisfy the Nyquist sampling theorem. From acomputational complexity point of view, it is beneficial to use an FFTor IFFT with a number of points that is an integer power of 2. Assumingthat NB-LTE will have 12 subcarriers, each having 15 kHz bandwidth, theFFT/IFFT size of the uplink can be 16 (=2⁴) points to minimizecomputational complexity. 16-point FFT and 15 kHz bandwidth for eachsubcarrier results in 240 kHz sample rate.

FIG. 1 is a functional block diagram that illustrates an example of atransmitter chain to facilitate the understanding of embodimentsdescribed herein. Use of a 240 kHz sample rate is assumed in thedescription of FIG. 1, but it should be remembered that other samplerates, also lower than 1.92 MHz, may also be used.

Modulator 1: This functional block is the modulator as in anycommunication system. Its maps input bit stream to constellationsymbols.

Serial to parallel converter 2: This functional block formats inputserial time-domain symbols into blocks, each of length M, where M is aninteger.

M-point FFT 3: This functional block converts each M paralleltime-domain symbols to M frequency-domain symbols.

Subcarrier mapping 4: This functional block maps each M frequency-domainsymbols to the specified subcarriers for transmission.

16-point IFFT 5: This functional block converts the mapped symbols onthe 16 subcarriers back to time domain.

CP adder 6: Since the signal has 240 kHz sampling rate every first OFDMsymbol of each slot have a CP of 10/8 samples and the rest of the OFDMsymbols 9/8 samples. The CP adder selects one CP sample that is the bestestimate of cyclic prefix. An example of this is given with reference toFIG. 2 below.

DAC CP control 7: This functional block controls the signal conversionof the DAC. When a CP is to be digital to analog converted it holds thesignal 10/8 times longer or 9/8 times longer than normal or doing anyother interpolation like e.g. put signal to 0 for 2/8 respectively 1/8of time and let reconstruction filter adopt to have a continuous signal,as illustrated in FIG. 2. In other words, the DAC CP control unit 7 isthus configured to adjust a sampling phase of the digital-to-analogconverter (DAC, described below) during cyclic prefixes.

DAC 8: Converts digital signal to analog signal and filters the outputsignal for reconstruction. DAC 8 has functionality to supportnon-uniform sampling controlled by the cyclic prefix insertion of theDAC CP control 7, in order to enable the adjustment of the samplingphase during the cyclic prefixes. DAC 8 can be of any known type, e.g.sigma-delta DAC or Nyquist DAC. The functionality to support non-uniformsampling may e.g. be achieved by providing a sampling clock signal widthadjustable phase to the DAC 8. For example, a clock-signal generator maybe configured to generate a plurality of sampling clock signals runningat the same frequency (e.g. 240 kHz) with a phase delays between thedifferent sampling clock signals. Adjusting the sampling phase may thusbe obtained by selecting which of the sampling clock signals iscurrently provided to the DAC 8. Other solutions, such as using aphase-locked loop (PLL) with adjustable phase, may be used as well.

RF unit 9: Up-converts the signal to RF for transmission.

FIG. 2 illustrates an example output waveform for the DAC 8. If thesample rate of the DAC 8 is denoted f_(s), the corresponding period timeT is 1/f_(s). The center part of FIG. 2 illustrates OFDM symbol 0, i.e.the first OFDM symbol of a slot. A copy of the last sample of the symbolis selected as the single sample CP according to this example, asindicated by the dashed arrow 50. As an example, a zero with length 2/8T is inserted in order to account for the varying CP length, which is10/8 T in the first symbol. For the other symbols of the slot, thelength of the zero is 1/8 T, for which the CP length is 9/8 T. Thesesinserted zeroes, which are inserted just before the cyclic prefixes inthe example, are indicated by reference sign 60 in FIG. 2. Thediscussion above assumes that f_(s)=240 kHz. For other sampling rates,which are equivalent to other values of T, the numbers would bedifferent. For example, for a 480 kHz sampling rate, which means the Tis half of that for 240 kHz, the inserted zeroes would be of lengths 2/4T (first symbol) and 1/4 T (other symbols). Similarly, for a 960 kHzsample rate, which means that T is half of that for 480 kHz, theinserted zeroes would be of lengths 2/2 T (first symbol) and 1/2 T(other symbols). Furthermore, higher sampling rates than 240 kHz enablesthe use of more than a single sample CP. For example, for 480 kHz, acopy of the two last samples of the symbol may be used in the CP, andfor 960 kHz, a copy of the four last samples of the symbol may be usedin the CP. The disclosure is not limited to the above example ofgenerating the signal values for the CP. The important point forenabling reducing the sampling rate (e.g. below 1.92 MHz in the NB IoTexample considered herein) is the adjustment of the sampling phaseduring the cyclic prefix. In the example in FIG. 2, this sampling phaseadjustment is obtained by delaying the first sampling instant in thesymbol by 2/8 T (for the first symbol in the slot) or 1/8 T (for theother symbols in the slot). Delaying in this context should beinterpreted as compared with if no sampling phase adjustment had beenmade. The sampling instances are indicated with crosses on the dashedline in FIG. 2. For a 480 kHz sampling rate, the sampling phaseadjustment may be obtained by the first sampling instant in the symbolby 2/4 T (for the first symbol in the slot) or 1/4 T (for the othersymbols in the slot). For a 960 kHz sampling rate, the sampling phaseadjustment may be obtained by the first sampling instant in the symbolby 2/2 T (for the first symbol in the slot) or 1/2 T (for the othersymbols in the slot).

In accordance with the disclosure above, FIG. 3 illustrates a method ina transmitter circuit of generating a signal comprising a first sequenceof an integer number N_(sym) of OFDM symbols, which are to betransmitted within a frequency sub band of a second sequence of N_(sym)OFDM symbols. The first sequence of OFDM symbols and the second sequenceof OFDM symbols are to be transmitted with the same timing. A firstcyclic prefix of the second sequence of OFDM symbols has a firstduration, or “length”. A second cyclic prefix of the second sequence ofOFDM symbols has a second duration, or “length”. The second duration isshorter than the first duration, such that in order to generate both thefirst and the second cyclic prefix with an integer number of equidistant(i.e. uniformly spaced) samples, a first sampling rate is required.Thus, the first sampling rate is the minimum required sampling rateneeded in order to generate both the first and the second cyclic prefixwith an integer number of equidistant samples. Higher sampling rates,such as integer multiples of the first sampling rate, may of course alsobe used for generating both the first and the second cyclic prefix withan integer number of equidistant samples. For example, the secondsequence of OFDM symbols may be transmitted in a Third-GenerationPartnership Project (3GPP) Long-Term Evolution (LTE) system andcorrespond to a slot. In that context, the first sequence of OFDMsymbols may be the OFDM symbols in corresponding slot of the NB IoTsystem used as an example herein. As discussed above, the first samplingrate may be 1.92 MHz. The method illustrated in FIG. 3 comprisesgenerating 100 the signal comprising the first sequence of OFDM symbolsat a second sampling rate, lower than the first sampling rate. Asdiscussed above, the second sampling rate may e.g. be 240 kHz. However,other sampling rates lower than 1.92 MHz may also be beneficially used,such as but not limited to 480 kHz, or 960 kHz. A benefit of 240 kHz,480 kHz, and 960 kHz is that these sample rates result in FFT and IFFTprocessing with the number of points being an integer power of two,which is advantageous in terms of computational complexity as mentionedabove. The method illustrated in FIG. 3 also comprises adjusting 110 asampling phase during cyclic prefixes. Thereby, a lower sampling ratethan the first sampling rate can be used despite that the cyclicprefixes have lengths which are non-integer multiples of the period atthe second sampling frequency.

In some examples, in line with what has been described above withreference to NB IoT, a subcarrier spacing of the OFDM symbols of thefirst and second sequences of OFDM symbols are 15 kHz, the firstduration is 160/30.72 μs and the second duration is 144/30.72 μs. Insome examples, also in line with what has been described above withreference to NB IoT, OFDM symbols of the first sequence of OFDM symbolshave 12 subcarriers.

In some examples, also in line with what has been described above withreference to NB IoT, the first cyclic prefix is an initial cyclic prefixof the second sequence of OFDM symbols, and all subsequent cyclicprefixes of the second sequence of OFDM symbols has the second duration.

Adjusting the sampling phase may comprise setting an initial sampleinstant during an OFDM symbol to occur a non-integer multiple of periodsat the second sampling rate after a final sample instant of a precedingOFDM symbol, e.g. as illustrated in FIG. 2.

Optionally, the method illustrated in FIG. 3 may comprise selecting 120a single sample value as a cyclic prefix for each OFDM symbol.Alternatively, this may be performed in a separate process. Asillustrated in FIG. 2, the single sample value may be selected equal tothe final sample value of the OFDM symbol. As mentioned above, more thanone sample may be used for the cyclic prefix depending on the value ofthe second sampling rate.

Also in accordance with the disclosure above, FIG. 4 illustrates atransmitter circuit 200 for generating a signal comprising a firstsequence of an integer number N_(sym) of OFDM symbols, which are to betransmitted within a frequency sub band of a second sequence of N_(sym)OFDM symbols. The first sequence of OFDM symbols and the second sequenceof OFDM symbols are to be transmitted with the same timing. A firstcyclic prefix of the second sequence of OFDM symbols has a firstduration, or “length.” A second cyclic prefix of the second sequence ofOFDM symbols has a second duration, or “length”. The second duration isshorter than the first duration, such that in order to generate both thefirst and the second cyclic prefix with an integer number of equidistantsamples, a first sampling rate is required. For example, the secondsequence of OFDM symbols may be transmitted in a 3GPP LTE system andcorrespond to a slot. In that context, the first sequence of OFDMsymbols may be the OFDM symbols in corresponding slot of the NB IoTsystem used as an example herein. As discussed above, the first samplingrate may be 1.92 MHz. The transmitter circuit 200 illustrated in FIG. 4comprises a DAC 8 configured to generate the signal comprising the firstsequence of OFDM symbols at a second sampling rate, lower than the firstsampling rate. This DAC corresponds to the DAC in FIG. 1, and thereforethe same reference number 8 is used in both figures. As discussed above,the second sampling rate may e.g. be 240 kHz. However, other samplingrates lower than 1.92 MHz may also be beneficially used, such as but notlimited to 480 kHz, or 960 kHz.

Furthermore, the transmitter circuit 200 illustrated in FIG. 4 comprisesa control unit 205 configured to adjust a sampling phase of the DAC 8during cyclic prefixes. Thereby, a lower sampling rate than the firstsampling rate can be used despite that the cyclic prefixes have lengthswhich are non-integer multiples of the period at the second samplingfrequency. Thus, the control unit 205 may be configured to perform thefunction of the functional unit 7 in FIG. 8. In some examples, thecontrol unit 205 may also be adapted to select a single sample value asa cyclic prefix for each OFDM symbol. Thus, the control unit 205 may beconfigured to perform the function of functional unit 6 in FIG. 1.Alternatively, this may be performed in some other unit. As illustratedin FIG. 2, the single sample value may be selected equal to the finalsample value of the OFDM symbol. As mentioned above, more than onesample may be used for the cyclic prefix depending on the value of thesecond sampling rate.

In some examples, in line with what has been described above withreference to NB IoT, a subcarrier spacing of the OFDM symbols of thefirst and second sequences of OFDM symbols are 15 kHz, the firstduration is 160/30.72 μs and the second duration is 144/30.72 μs. Insome examples, also in line with what has been described above withreference to NB IoT, OFDM symbols of the first sequence of OFDM symbolshave 12 subcarriers.

In some examples, also in line with what has been described above withreference to NB IoT, the first cyclic prefix is an initial cyclic prefixof the second sequence of OFDM symbols, and all subsequent cyclicprefixes of the second sequence of OFDM symbols has the second duration.

The control unit 205 may be configured to adjust the sampling phase bysetting an initial sample instant during an OFDM symbol to occur anon-integer multiple of periods at the second sampling rate after afinal sample instant of a preceding OFDM symbol, e.g. as illustrated inFIG. 2.

As illustrated in FIG. 4, the transmitter circuit 200 may comprise an RFunit 9, e.g. corresponding to the RF unit 9 in FIG. 1. Furthermore, thetransmitter 200 may comprise a baseband (BB) unit 210. The BB unit 210may e.g. be configured to perform the functions of functional units 1-5in FIG. 1. In some examples, the BB unit 210 may be configured toperform the function of functional unit 6 in FIG. 1, i.e. to select thesample value of the cyclic prefix. In some examples, the BB unit 210 andthe control unit 205 may be implemented in the same hardware unit. Inother examples, they may be implemented as separate different hardwareunits. Said hardware units may be application specific hardware units,programmable hardware units, or any combination thereof.

It should be noted that it is the first sequence of OFDM symbols that isto be transmitted by the transmitter circuit 200. The second sequence ofOFDM symbols may be transmitted by one or more transmitters, e.g. in oneor more other devices. However, the first and the second sequences ofOFDM symbols have the same timing to facilitate coexistence.

FIG. 5 illustrates an environment wherein the method and transmittercircuit disclosed herein can be employed. In FIG. 5, a machine-typecommunication (MTC) device 300 communicates wirelessly with a networknode 310 of a cellular communication system. The communication may takeplace, for example, using NB IoT as described above. The network node310 may thus be an eNodeB for NB IoT, according to some examples. TheMTC device 300 and the network node 310 are nonlimiting examples ofelectronic devices that may comprise the transmitter circuit 200.

The present disclosure has been presented above with reference tospecific examples. However, other implementations than the abovedescribed are possible. Different method steps than those describedabove, performing the method by hardware or software, may be provided.The different features and steps of the examples may be combined inother combinations than those described. For example, the transmitterdoes not have to be partitioned exactly as the functional block diagramin FIG. 1.

What is claimed is:
 1. A method in a transmitter circuit of generating aNarrow-Band Internet-of-Things (NB-IoT) signal comprising a sequence ofan integer number of OFDM symbols, wherein the method comprises:generating the signal comprising the sequence of OFDM symbols at asampling rate lower than 1.92 MHz; and adjusting a sampling phase duringcyclic prefixes of the OFDM symbols.
 2. The method of claim 1, wherein afirst cyclic prefix for one or more of the sequence of OFDM symbols hasa duration of 160/30.72 μs, a second cyclic prefix for one or more ofthe sequence of OFDM symbols has a second duration of 144/30.72 μs, anda subcarrier spacing of the OFDM symbols of the sequence of OFDM symbolsis 15 kHz.
 3. The method of claim 1, wherein a first cyclic prefix forone or more of the sequence of OFDM symbols has a first duration, asecond cyclic prefix for one or more of the sequence of OFDM symbols hasa second duration, and the second duration is shorter than the firstduration, such that in order to generate both the first and the secondcyclic prefix with an integer number of equidistant samples, a firstsampling rate of 1.92 MHz would be required.
 4. The method of claim 1,wherein the first cyclic prefix is an initial cyclic prefix of thesequence of OFDM symbols, and all subsequent cyclic prefixes of thesequence of OFDM symbols have the second duration.
 5. The method ofclaim 4, wherein OFDM symbols of the first sequence of OFDM symbols have12 subcarriers.
 6. The method of claim 5, wherein the sequence of OFDMsymbols is transmitted in a slot.
 7. The method of claim 1, whereinadjusting the sampling phase comprises setting an initial sample instantduring an OFDM symbol to occur a non-integer multiple of periods at thesecond sampling rate after a final sample instant of a preceding OFDMsymbol.
 8. A transmitter circuit for generating a Narrow-BandInternet-of-Things (NB-IoT) signal comprising a sequence of an integernumber of OFDM symbols, wherein the transmitter circuit comprises adigital-to-analog converter configured to generate the signal comprisingthe sequence of OFDM symbols at a sampling rate lower than 1.92 MHz; anda control circuit configured to adjust a sampling phase of thedigital-to-analog converter during cyclic prefixes of the OFDM symbols.9. The transmitter circuit of claim 8, wherein a first cyclic prefix forone or more of the sequence of OFDM symbols has a duration of 160/30.72μs, a second cyclic prefix for one or more of the sequence of OFDMsymbols has a second duration of 144/30.72 μs, and a subcarrier spacingof the OFDM symbols of the sequence of OFDM symbols is 15 kHz.
 10. Thetransmitter circuit of claim 8, wherein a first cyclic prefix for one ormore of the sequence of OFDM symbols has a first duration, a secondcyclic prefix for one or more of the sequence of OFDM symbols has asecond duration, and the second duration is shorter than the firstduration, such that in order to generate both the first and the secondcyclic prefix with an integer number of equidistant samples, a firstsampling rate of 1.92 MHz would be required.
 11. The transmitter circuitof claim 8, wherein the first cyclic prefix is an initial cyclic prefixof the sequence of OFDM symbols, and all subsequent cyclic prefixes ofthe sequence of OFDM symbols have the second duration.
 12. Thetransmitter circuit of claim 11, wherein OFDM symbols of the firstsequence of OFDM symbols have 12 subcarriers.
 13. The transmittercircuit of claim 12, wherein the transmitter is configured to transmitthe sequence of OFDM symbols in a slot.
 14. The transmitter circuit ofclaim 8, wherein adjusting the sampling phase comprises setting aninitial sample instant during an OFDM symbol to occur a non-integermultiple of periods at the second sampling rate after a final sampleinstant of a preceding OFDM symbol.
 15. An electronic device comprisingthe transmitter circuit of claim
 8. 16. The electronic device of claim15, wherein the electronic device is a machine-type communication (MTC)device.
 17. The electronic device of claim 15, wherein the electronicdevice is a network node for a cellular communication system.